Patent Literature 1 discloses a gas sensor which takes an exhaust gas discharged from an on-vehicle engine as an object of a detection and detects a NOx concentration of the exhaust gas. The gas sensor (NOx sensor) includes a pump cell, a monitor cell, and a sensor cell each having a solid electrolytic element and a pair of electrodes disposed on the solid electrolytic element and uses the pump cell to discharge an oxygen in the exhaust gas introduced into a chamber and uses the sensor cell to detect the NOx concentration of the exhaust gas after the oxygen has been discharged. A deterioration diagnosing apparatus which diagnoses a deterioration of the gas sensor forcibly changes an applied voltage that is applied to the pump cell and diagnoses the deterioration of the gas sensor based on a change of an output of the sensor cell caused by a switching of the applied voltage. In the deterioration diagnosing apparatus, the output of the sensor cell is corrected based on an amount of a change of the applied voltage and an amount of the change of the output of the sensor cell.
Patent Literature 2 discloses various techniques to perform an abnormality diagnosis of the gas sensor of the above kind. For example, in the techniques disclosed in Patent Literature 2, a deterioration detection cell is provided in the vicinity of a sensor cell and an output corresponding to an output of a deterioration product of the sensor cell is obtained by the deterioration detection cell. It is determined whether or not a NOx sensor has deteriorated by comparing an output of the sensor cell with the output of the deterioration detection cell.
In Patent Literature 1, however, even when a constant voltage is applied to the pump cell, an amperage of a current flowing through the pump cell may be an amperage that is undesired. Further, it is possible that an accuracy of a deterioration diagnosis of the sensor cell may be decreased or an output correction of the sensor cell may be improperly performed. For example, in the pump cell, a pump cell current that is the current flowing through the pump cell (i.e., an amount of the oxygen discharged by the pump cell) is changed depending on the deterioration or an activation state, so as to affect a residual oxygen concentration that is a concentration of the oxygen that is residual in the chamber. The output of the sensor cell is changed by an effect applied to the residual oxygen concentration, with a result that the deterioration diagnosis or the output correction may be improperly performed.
In Patent Literature 2, since it is necessary to provide the deterioration detection cell other than the pump cell, the monitor cell, and the sensor cell to detect a sensor deterioration, a structure becomes complicated.